<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>oi-li</title>
</head>
<body>
  <ol>
    <li>First</li>
    <li>Second</li>
    <li>Third</li>
    <li>Forth</li>
    <li>Fifth</li>
  </ol>
</body>
<style>
  * {
    background: #00a8cc;
    color: #ffe75e;
    font-weight: bold;
    font-size: 50px;
  }

  li {
    margin: auto auto auto 30px;
  }

  /* nth-child 满足 x=2n+1(n∈N*)的元素，从0开始计数 */
  /* 它是伪类 */
  li:nth-child(2n+1) {
    color: white
  }
</style>
</html>